The present invention relates to the removal of adsorbed NH3 during the process for separating different types of particulates, e.g., carbon from fly ash, using an improved inclined fluidized bed approach and acoustic enhancement.
In a typical pulverized coal power plant, 80 percent of the ash is carried from the boiler as fly ash and is removed from the flue gas in an electrostatic precipitator, fabric filter or wet scrubber. As a result, U.S. electric utilities spend about $1 billion annually to dispose of most of the 75 million tons of ash removed from their pulverized coal burning plants. For this reason, there has been worldwide activity for many years on the development of ways of utilizing fly ash as an alternative to ash disposal. One of the high volume end uses which has found commercial acceptance is the substitution of fly ash for some of the cement used in concrete.
However, in addition to containing a variety of inert minerals, fly ash may also contain undesirable amounts of unburned carbon. High levels of unburned carbon in fly ash are common with bituminous coals burned in both corner-fired and wall-fired pulverized coal boilers. High levels of unburned coal are even more severe when low NOx burners are used for NOx control. Many utilities who wish to sell their fly ash for use in concrete must reduce these levels of fly ash carbon at high cost. Although over 56 million tons of cement were used to produce concrete in the U.S. in 1991, only approximately 7 million tons of fly ash went into U.S. concrete due to the cost of carbon removal.
One of the standard laboratory tests for the amount of unburned carbon in fly ash is the so-called loss-on-ignition test (LOI). In this disclosure, the terms LOI and unburned carbon will be used interchangeably. The amount of LOI is influenced by the size distribution of coal leaving the coal pulverizers, the combustion conditions in the furnace and the design of he furnace and the burners. For utility boilers burning bituminous coal, utilities have traditionally tried to maintain the LOI in the fly ash to below 6 weight percent. This is done to prevent excessively large losses in boiler efficiency. But for many boilers, this has not been an easy goal to meet. For those utilities who wish to sell their fly ash for use in concrete, it is necessary to maintain the fly ash carbon to even lower levels, with limits of 3 to 5 percent carbon content usually imposed by the concrete manufacturers. Utilities are now installing low NOx burners on their boilers for NOx control. However, as described above, this escalates the problem of fly ash LOI. Test data resulting from recent low NOx installations show that operating a boiler with low NOx burners invariably results in an increase in fly ash LOI, sometimes two-fold in extent.
Thus, there remains a commercial need for an industrial process which beneficiates fly ash in an economical manner, thus making it more amenable to reuse in concrete and otherwise. The problem lies in the separation and removal of carbon from fly ash.
An improved continuous process for cleaning coal is disclosed in U.S. Pat. No. 5,197,398. This process, referred to as D-CoP, relies on an inclined fluidized bed (shown in FIG. 4 herein). This type of bed resembles a long, nearly horizontal, table, with fluidizing air passing vertically upward through a distributor causing bubbling to occur in the bed material. The coal and magnetite, which are added to the bed at one end via coal feed and magnetite feed, flow along the length of the bed, and as they do so, the pyrite and other minerals sink downward through the layer of particles. The clean coal with some magnetite is then separated from the coal refuse at the discharge end. The D-CoP process relies on the use of a bubbling fluidized bed to achieve separation of particles based on differences in density. The bubbles, which are formed at the distributor located at the bottom of the bed, act as pumps. They carry material upward from the bottom of the bed, and at the same time, provide a mechanism by which relatively dense particles near the top of the bed can move downward. Thus, the ability to achieve stable bubbling fluidization is central to the good separation efficiencies which are achieved by D-CoP when used for cleaning coal.
In order to apply this fluidized bed approach to fly ash, it is necessary to achieve good bubbling fluidization. However, fly ash particles are relatively small in size, with mean particle diameters which are typically less than 15 to 20 microns. Particles in this size range do not fluidize well in the bubbling mode. Instead, the particles tend to clump together, causing an unsteady slugging type of fluidization. A solution to this is described in U.S. Pat. No. 5,996,808 to Levy et al., which discloses a process for separating different types of particulates, e.g., carbon from fly ash, using an improved inclined fluidized bed approach with acoustic enhancement. According to the ""808 patent, the dramatic effect of high intensity sound on bubbling with particulates such as fly ash enhances the beneficiation process. The sound affects characteristics such as bubble size distribution, shape, and wake configuration and bubble frequency, and it agitates the bed material to disrupt the interparticle forces. This promotes more uniform and consistent bubbling in the bed and thus enhances the segregation processes in the bed which are needed to achieve efficient separation of carbon. FIG. 1 is an illustration from the ""808 patent showing how a loudspeaker 270 may be oriented with respect to the fluidized bed to produce the requisite acoustic excitation. The speaker(s) 270 may be any conventional loudspeaker(s) capable of generating the requisite Sound Pressure Level (SPL) in dB and frequencies (Hz). The speaker(s) 270 should be mounted above the bed and are preferably operated at power levels which generate values of SPL in excess of 140 dB at the free surface of the fluidized bed, and at frequencies of approximately 90 Hz (although this may vary).
In the enhanced process, ammonia (NH3) is present in fly ash from some power plants due to NH3 injection for NOx control or for conditioning the flue gas to improve electrostatic precipitator performance. The presence of NH3 on the ash can create a safety hazard for workers who came in contact with the ash and can restrict use of ash in concrete production and other applications. It would be greatly advantageous to further supplement the process for separating carbon from fly ash during the enhanced fluidized bed approach with acoustic enhancement as described in U.S. Pat. No. 5,996,808 to Levy et al. to also remove ammonia (NH3).
In accordance with the above, it is an object of the present invention to provide an improved industrial process for removing ammonia from fly ash during the use of an enhanced fluidized bed with acoustic enhancement as described in U.S. Pat. No. 5,996,808 to Levy et al., thereby making the fly ash and the carbon removal process safer for the production of concrete, etc.
It is another object to remove ammonia as described above in a simple and efficient manner by preheating the fluidizing air of the inclined bed to temperatures of 300 to 500xc2x0 F., and using this preheated air to drive off any adsorbed ammonia.
It is a further object to present a flexible ammonia stripping application as described above that can be employed independently of the carbon removal such that the entire inclined bed can be dedicated to the ammonia removal or, alternatively, the processes can be integrated such that ammonia removal is accomplished in a first stage and carbon separation in a second stage of processing.
The above and other objects and advantages of the invention will become more readily apparent on examination of the following description, including the drawings, in which like reference numerals refer to like parts. Generally, the improvement disclosed herein is a process for removing ammonia from fly ash during processing on an inclined fluidized bed. The process begins by introducing a mixture of particulates having ammonia adsorbed therein into an inclined fluidized bed. Concurrently, a fluidizing gas is pre-heated and is also introduced into the fluidized bed. The mixture is then processed along the fluidized bed with the pre-heated fluidizing gas to achieve bubbling fluidization of the particles. This causes segregation by which a dense fraction settles downward and a light fraction rises upward in the bed. Ammonia in the particles is desorbed by the pre-heated fluidizing gas. The fluidizing gas is then scrubbed after the processing step to remove the desorbed ammonia. The process may also include the use of acoustic enhancement whereby an acoustic field is imposed on the fluidized bed to improve fluidization and segregation of the particles and to increase the efficacy of ammonia removal.
In addition, a heat exchanger or electric heating elements immersed in the fluidized bed can be used to heat the ash to the temperatures needed to desorb ammonia from the ash. Furthermore, the energy efficiency of the process can be improved by recovering heat from the hot ash discharged from the fluidized bed and/or from the exhaust stream of hot fluidizing gas, for reuse during preheating steps.